feat: add DROP DEFAULT (#246)

Signed-off-by: Yihai Lin <yihai-lin@foxmail.com>

proto/greptime/v1/ddl.proto

@@ -114,9 +114,14 @@ message AlterTableExpr {
     UnsetTableOptions unset_table_options = 11;
     SetIndex set_index = 12;
     UnsetIndex unset_index = 13;
+    DropDefaults drop_defaults = 14;
   }
 }
 
+message DropDefault {
+  string column_name = 1;
+}
+
 message SetIndex {
   oneof options {
     SetFulltext fulltext = 1;
@@ -165,6 +170,10 @@ message AddColumns {
   repeated AddColumn add_columns = 1;
 }
 
+message DropDefaults {
+  repeated DropDefault drop_defaults = 1;
+}
+
 message DropColumns { repeated DropColumn drop_columns = 1; }
 
 message ModifyColumnTypes { repeated ModifyColumnType modify_column_types = 1; }

proto/greptime/v1/region/server.proto

@@ -140,6 +140,7 @@ message AlterRequest {
     UnsetTableOptions unset_table_options = 9;
     SetIndex set_index = 10;
     UnsetIndex unset_index = 11;
+    DropDefaults drop_defaults = 12;
   }
   // The version of the schema before applying the alteration.
   uint64 schema_version = 4;